1. Field of the Invention
The present invention relates to an inspection method and an inspection apparatus for inspecting various photomasks such as a binary type photomask, a half-tone type photomask, and a tri-tone type photomask.
2. Description of Related Art
With the miniaturization of semiconductor devices, there is a demand for development of an inspection apparatus capable of detecting a fine defect present in a photomask with high sensitivity. As a related-art inspection apparatus that detects a defect present in a photomask, an inspection apparatus is known in which a transmission image and a reflection image of a photomask are individually picked up and a defect is detected based on the transmission image and the reflection image (see, for example, U.S. Pat. No. 7,664,310). Such a known inspection apparatus has a configuration in which an illumination beam output from a light source is projected toward a pattern forming surface of a photomask; the reflected beam reflected by the pattern forming surface of the photomask is received by a first detector; and the transmitted beam transmitted through the photomask is received by a second detector that is disposed on the opposite side of the light source. Output signals output from the first and second detectors are supplied to a processor. In the processor, Cartesian coordinates are provided by using an intensity T of the transmitted light and an intensity R of the reflected light, and T-R data on various photomasks are accumulated to form a T-R map. A portion of data outside a range defined by the envelope of the generated T-R map is determined to be defective.
Another known inspection apparatus has the following configuration. In the inspection apparatus, a reflected illumination beam is projected toward a pattern forming surface of a photomask to be inspected and a transmitted illumination beam is projected toward the back surface thereof; a composite image obtained by optically adding a transmission image and a reflection image of a region of the photomask is picked up; and a defect is detected based on the composite image (see, for example, U.S. Pat. No. 7,046,352 and Japanese Unexamined Patent Application Publication Nos. 2007-132729 and 2008-96296).
As a still further inspection apparatus, an inspection apparatus is known in which a transmission image of a photomask and a composite image of a transmission image and a reflection image are picked up, and a defect is detected based on the transmission image and the composite image (see, for example, Japanese Unexamined Patent Application Publication No. 2008-190938). In such a known inspection apparatus, the field of view of an objective lens is divided into two regions. The transmission image of the photomask is formed in one of the divided regions of the field of view, and the composite image of the transmission image and the reflection image is formed in the other region of the field of view.
A photomask has a structure in which a light-shielding pattern of a molybdenum silicide film or a metallic chromium film is formed on a quartz substrate. The pattern has been miniaturized with the miniaturization of devices. However, with the miniaturization of the pattern, the diffraction effect on an edge portion of the pattern becomes more prominent, and there is a strong demand for ameliorating the deterioration in detection sensitivity at the pattern edge portion. Specifically, the pattern edge portion forms a kind of optical step. Accordingly, illumination light incident in the vicinity of the pattern edge is affected by the diffraction effect due to the step and reflected light output from the pattern is not incident on a photodetector, with the result that a malfunction occurs due to a decrease in resolution of the edge portion of the pattern. In particular, transmitted illumination light is relatively slightly affected by the diffraction effect due to the pattern edge, while reflected illumination light is greatly affected by the diffraction effect. As a result, in the inspection apparatus, which individually picks up a reflection image and a transmission image to detect a defect, the resolution of the reflection image significantly decreases and a malfunction occurs due to the deterioration in detection sensitivity with respect to a foreign matter defect present in the vicinity of the edge on the pattern.
In the inspection apparatus that projects illumination beams toward the front surface and the back surface of a photomask at the same time and picks up the composite image of the transmission image and the reflection image of the photomask, the transmitted light transmitted through the photomask is partially incident on the photodetector that receives the reflected light by the diffraction effect. This is advantageous in that the effect due to the diffraction effect at the edge of the light-shielding pattern is reduced and in that minute foreign matter defects present in the vicinity of the edge of the pattern can be satisfactorily detected. Further, the transmission image of the photomask is relatively slightly affected by the diffraction effect, which is beneficial for inspection of a defect present in a light-transmitting portion (an area in which no pattern is formed). However, there is a drawback that the diffraction effect cannot also be avoided in the composite image, which is obtained by synthesizing the transmission image and the reflection image, and a low-luminance image (hereinafter, referred to as “a drop image”) in which the luminance of the image is locally reduced in the vicinity of the edge of the pattern due to the diffraction effect is formed. Such drop images are prominently generated during an inspection of a half-tone type phase shift mask (EPSM). The low-luminance image causes a pseudo defect. When a threshold level for detection of a defect is set to a high level, the low-luminance image is detected as a pseudo defect and a malfunction occurs in which a normal region is determined as a defect. On the other hand, when the threshold level is set to a low level so as to prevent the drop image from being detected as a defect, a minute defect to be detected cannot be detected and a malfunction occurs in which the sensitivity for detection of a defect deteriorates.
Furthermore, in the inspection apparatus that detects a defect by using the composite image, it is necessary to set an appropriate amount of reflected light at the pattern edge. Therefore, the following problem arises. That is, constraints are imposed on the intensity of the transmitted illumination light and the illumination intensity of the transmitted illumination light is shifted from an optimum illumination intensity. Specifically, when it is primarily intended to detect a foreign matter defect on a pattern, the intensity of the transmitted illumination light is shifted from an optimum value. Further, when it is primarily intended to detect a defect in the light-transmitting portion (quartz substrate), the intensity of the transmitted illumination light is set to the optimum value, but a malfunction occurs in which the sensitivity for detection of a defect in the vicinity of the edge portion of the pattern portion deteriorates.
The inspection apparatus that individually picks up a transmission image and a composite image of a transmission image and a reflection image of a photomask has an advantage that a defect inspection using the transmission image of the photomask and a defect inspection using the composite image can be individually carried out. In other words, since the diffraction effect due to the pattern edge on the transmitted illumination light is relatively small, a defect present in the light-transmitting portion can be detected with high sensitivity even when the pattern is miniaturized. Further, the resolution in the pattern edge portion of the composite image is relatively high with respect to a defect present in the pattern portion, which is advantageous in that a defect present in the vicinity of the edge portion of the pattern can be detected with high detection sensitivity. There is another advantage that the transmission image inspection and the composite image inspection can be carried out by setting an optimum illumination light intensity. This makes it possible to achieve an advantage that a defect inspection can be carried out more satisfactorily than in the above-mentioned inspection apparatus that carries out the inspections based only on the composite image.
However, the diffraction effect due to the pattern edge causes the low-luminance image (drop image), in which the luminance is locally reduced in the vicinity of the edge of the pattern portion in the composite image, to be formed, and especially, the low-luminance image is prominently generated during the inspection of the EPSM. In this regard, the following problem arises. That is, a pseudo defect is generated due to the presence of the low-luminance image, or the presence of the low-luminance image makes it difficult to set the detection sensitivity to a high level. Therefore, if a defect detection method in which the effect of the drop image is eliminated can be achieved, a defect present in the vicinity of the edge of the pattern portion can be detected with high sensitivity even when the light-shielding pattern is miniaturized.
An object of the present invention is to provide a defect inspection method and an inspection apparatus in which the diffraction effect due to a pattern edge is reduced.
Another object of the present invention is to provide an inspection method and an inspection apparatus which are not affected by a drop image formed in the vicinity of a pattern edge in a defect inspection in which a transmission image and a composite image obtained by synthesizing a transmission image and a reflection image of a photomask are individually picked up and a defect is detected based on the transmission image and the composite image.